1. Field of the Invention
The present invention relates generally to thin film, multilayer coatings, and more particularly to low emissivity coatings.
An "induced transmission filter" employs a single film of silver bounded by dielectric multilayer stacks of alternating high and low index materials. See P. H. Berning and A. F. Turner, "Induced Transmission in Absorbing Films Applied to Band Pass Filter Design", J. Opt. Soc. Am., Vol. 47, No. 3, pp. 230-9 (1957). This filter was originally conceived as a means of making narrow band interference filters with extended blocking regions due to the metal reflectivity. Transmission is "induced" through the metal film in a narrow range by the antireflection coating effect of the dielectric layers.
The now widely-used low emissivity (E) coatings are simply an induced transmission filter with a wide bandpass region, i.e., the entire visible spectrum. The structure used to achieve this result has a very thin silver film approximately three times thinner than the original Berning and Turner narrow band designs. As such, it requires only a single layer of dielectric material, preferably a high refractive index material such as titanium oxide or zinc oxide.
Low E coatings are used, for example, to reduce heat loss caused by radiation through the outside windows of buildings. This is accomplished by designing the coatings to provide high reflectivity for longer wavelength infrared radiation, typically a wavelength of about 10 micrometers. The coatings are also designed to provide high transmission but low reflection for visible radiation (light). It is also desirable that the color of the transmitted visible light not be affected by the coating. Typical performance values for such coatings are: a reflection for infrared radiation greater than about 90 percent, a transmission for visible radiation greater than about 75 percent, and a reflection for visible radiation of about 4 percent.
In warmer climates, it is also important to reduce the heat input into a building through its outside windows. The total heat input through the windows is apportioned in the electromagnetic spectrum of radiation as follows: less than about 5 percent is at wavelengths less than 425 nanometers (nm), i.e., in the ultraviolet region of the spectrum; about 40 percent is at wavelengths between 425 nm and 675 nm, i.e., the visible region of the spectrum; and about 45 percent is between wavelengths of 675 and 1200 nm, generally referred to as the near infrared (NIR) spectral region.
In low E coatings for use in warm climates, it is desirable to suppress light in the NIR region, preferably by reflection and without affecting the visible light transmission properties. Suppression of visible light transmission, of course, also reduces solar heat input and is sometimes used to this end. The compromise between heat and visible light transmission reduction is usually determined by aesthetic and other energy considerations, such as an increase in the illumination costs resulting from a reduction in visible light input.
2. Description of the Related Art Layer Systems with One Metal Film
The basic low E structure is a three layer system. It is shown schematically in FIG. 1. The structure 20 comprises a layer of transparent dielectric material 22 in contact with a glass, for example, substrate 24. A layer of metal 26, which is highly reflective at infrared wavelengths but thin enough to be partially transparent to visible radiation, is disposed between dielectric layer 22 and another transparent dielectric layer 28.
The metal layer 26 may be silver with a thickness about 10 nm. The dielectric layers 22 and 28 are typically zinc oxide, titanium oxide, or tin oxide. The dielectric layers may have a refractive index greater than about 1.9. The optical thickness of the dielectric layers may be about one-quarter of wavelength at a wavelength of 520 nm, i.e., in the middle of the visible spectrum. The dielectric films or layers reduce the reflection of visible light from the metal film, and thus increase the transmission of visible light through the structure.
In other structures, such as structure 30 of FIG. 2, it is not unusual to use much thinner dielectric layers 32, 34. Layers 32, 34 may be about 10 nm thick and are disposed on opposite sides of metal layer 26. The additional dielectric layers 32, 34 promote adhesion of the thicker dielectric layers 22 and 28, and/or prevent oxidation of metal film 26 during the deposition of dielectric layers 22, 28. The thin layers 32, 34 are referred to as barrier layers or adhesion layers. They are sufficiently thin so that they do not interfere with the passage of visible radiation. The optical principle of structure 30, a five film structure, is identical to that of three layer structure 20.
European patent application Serial No. 85113488.2, Publication No. 0 183 052, published Jun. 4, 1986, discloses the advantages of binder layers for solar control coatings with one metallic layer. This application also discloses the use of an alloy target to increase sputtering rates without a sacrifice in durability. The binder layers are added to prevent oxidation of the metallic layer. The binder layers improve the adhesion between the metallic and metal oxide layers. The binder layers are metal films which themselves become oxidized by the subsequent process steps. These oxidized binder layers then become part of the dielectric films surrounding the metallic layer. They are sufficiently thin so that they make no significant contribution to the interference design other than being part of the dielectric film. The optical system is simply: a dielectric/metal/dielectric (DMD) induced transmission filter.
The spectral performance of a silver film low E system was computed and is shown in FIGS. 3 and 4. The dielectric layers 22 and 28 were considered to be titanium oxide (TiO.sub.2) having thickness of 43.5 and 37 nm, respectively. The thickness of the silver metal layer was selected to be 10 nm. The performance in the visible and NIR regions is shown more clearly in FIG. 4. It can be seen that the system fulfills the requirement for IR reflection and high visible transmission (curve 38), but does not significantly reduce transmission (curve 40) in the NIR region of the spectrum.
Layer Systems with Two Metal Films
The induced transmission filter principle can be extended to include layer systems that have two or more metal films. In the simplest form, in which they would be used for low emissivity or solar control coatings, such systems would comprise five optically functional layers as follows: dielectric/metal/dielectric/metal/ dielectric (DMDMD). Some other layers may be included for adhesion or barrier purposes as already discussed. Some general references on the optical principles are discussed for a number of metal dielectric systems in Thin Film Optical Filters by MacLeod, Chap. 7, pp. 292-311, 2d ed. (1986). See also: P. H. Lissberger, "Coatings with induced transmission", Appl. Opt. Vol. 20, No. 1, pp. 95-104 (1981).
For instance, the structure 42 of FIG. 5 is designed to provide attenuation of the NIR wavelengths. It includes two metal layer 44, 46 separated by a transparent dielectric layer 48. Additionally, transparent dielectric films 45 and 50 are located on the outer surfaces of metal layer 44 and 46. As shown, such a structure has a minimum of five layers.
The metal layers 44, 46 may have a thickness of about 10 nm, and the dielectric layer 48 separating the metal layers may have an optical thickness of about one-half wavelength at a wavelength of 520 nm. The outer dielectric layers 45, 50 may have an optical thickness of about one-quarter wavelength at 520 nm. The materials used may be similar to those used in the three layer system of FIG. 1. Additional thinner layers may be employed as barrier or adhesion layers as described above.
The spectral performance of structure 42 has been computed and is shown in FIG. 6. The performance in the visible and NIR spectral regions is shown in FIG. 7. The dielectric layers 45, 48 and 50 were considered to be TiO.sub.2 having thickness of 32 nm, 65 nm and 31.2 nm, respectively. The metal layers 44 and 46 were considered to be silver each having a thickness of about 150 nm. As can be seen from FIGS. 6 and 7, the structure is quite effective in suppressing the NIR wavelengths (curve 43) without significantly reducing transmission of visible wavelengths (curve 44).
In U.S. Pat. No. 4,799,745, silver or a silver-gold alloy is utilized as the metal of preference. A symmetrical system is disclosed as an example and, although not specifically stated, it is implicit that the two metal films are of the same metal. It is also stated that the layers need not be the same thickness, and indeed in this five layer system, the metal layer thicknesses can be adjusted to give a lower reflection on one surface.
Characteristics of the Five Layer, Two Metal Layer System
A distinctive characteristic of structures with two metal layers, such as structure 42, is that the NIR rejection properties are connected, by virtue of the design principle, with the transmission bandwidth in the visible region of the spectrum. To illustrate this, the performance of three different five layer systems (two metal layers) using silver layers of various thickness has been computed. The thickness of the silver layer was made the same for both layers, and those thicknesses were set at 10, 15 and 20 nm. The result is shown in FIG. 8 wherein curves 52, 54 and 56 represent the transmission bandwidth for the thickness of 10, 15 and 20 nm, respectively. The dielectric layers were assumed to be titanium dioxide. The thickness of layers 45 and 50 was considered to be about 35 nm, and the thickness of the center dielectric layer was about 70 nm. The designs were optimized to provide the lowest reflection in the visible spectral region.
The reduction in the spectral bandwidth accompanied by a reduction in the peak transmission value as the silver layers are made thicker is very clear. The bandwidth reduction for the structure using 20 nm thick silver layers is sufficiently restricted such that light transmitted through the window would appear greenish in color.
These examples are offered to illustrate the principle of this design. It is recognized that further refinements based on different compromises between reflection and transmission are possible. These could be effected by employing, for example, silver layers of unequal thickness, different dielectric materials, and/or different dielectric layer thicknesses. It is also possible to extend the design principle to create systems with three metal layers and four dielectric layers or, in general, with any number (n) metal layers and n+1 dielectric layers.
It is an object of the present invention to provide a solar control coating having only one metal film component but offering high rejection in the NIR region. It is a further object of the present invention to provide solar control coatings which are sufficiently durable to be deployed on exposed glass surfaces.